Abrasive inserts for grinding bimetallic components

ABSTRACT

A method and apparatus for machining flat or planarizing a fire deck of a bimetallic engine block includes providing a grinding wheel by disposing a series of abrasive inserts at spaced locations along a circumference of a circular head, so that during rotation of the head, the inserts form a notional annular grinding ring. The head is preferably a conventional milling head and the inserts are sized and shaped to fit within receptacles adapted to receive conventional milling inserts. In an alternate embodiment, a second series of abrasive inserts may be disposed radially inward of the first series of inserts to form an inner notional annular grinding ring disposed concentrically with the outer ring. Inner and outer inserts preferably comprise a single layer of diamond abrasive brazed onto a metallic substrate. Fabrication of the inserts with a single layer of abrasive brazed onto a substantially planar grinding face may enable a conventional milling head and milling machine to grind bimetallic workpieces without the need for the guarding typically required for bonded abrasive grinding wheels.

This application is a Continuation-in-Part of U.S. patent applicationSer. No. 08,908/657, filed on Aug. 7, 1997, U.S. Pat. No. 5,951,378entitled “Method For Grinding Bimetallic Components”.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to abrasive tools, and more particularly toabrasive inserts adapted for use in grinding the surface of bimetallicengine blocks.

2. Background Information

As automakers push to reduce the weight of automobiles, the engine blockremains one of the heaviest single components. Manufacturing the engineblocks in a bimetallic manner, such as by fabricating the blocks fromaluminum and placing cast iron sleeves into the cylinder bores cansubstantially reduce the weight of the engine block relative toconventional cast iron engine blocks. An important aspect of the engineblock manufacturing process, however, is to provide the block with aflat or planarized upper surface or fire deck for mating with thecylinder head. Machining of conventional unimetallic engine blocks (i.e.cast iron) is generally accomplished by common machining processes suchas fly cutting or high speed milling utilizing hardened ceramic inserts,such as silicon nitride, tungsten carbide or polycrystalline diamond(PCD), on the milling head. This process using PCD inserts has also nowbeen adopted for use in machining bimetallic blocks. Althoughsatisfactory when utilized for unimetallic blocks, this approach tendsto produce undesirable results when used with blocks fabricated from twomaterials, one of which is soft, i.e., aluminum, and the other of whichis brittle, i.e., cast iron. When utilized to mill bimetallic parts, therelatively expensive PCD inserts tend to wear rapidly. Moreover, toinsure a smooth and flat surface, multiple passes with the millinginserts are typically utilized, although score lines may still be seen.Waviness also sometimes occurs in the surface of the fire deck. Theseproblems may be associated with, or exacerbated by, the differences inoptimal milling tool configuration for soft versus brittle materials.For example, most high-speed milling cutters made for softer materials,such as aluminum, operate most efficiently at substantially greater rakeangles than those used for harder materials such as cast iron. Clearanceangles, or the angle between the land and a tangent to the cutter fromthe tip of the tooth, also depend on the various work materials. Castiron typically requires values of 4 to 7 degrees, whereas soft materialssuch as magnesium, aluminum, and brass are cut efficiently withclearance angles of 10 to 12 degrees. (See, e.g., B. H. Amstead et al.Manufacturing Processes, 1977, pp. 555-556).

One solution to this problem has been to countersink the cast ironsleeves to the depth to which the aluminum is to be removed. Oncecountersunk, the aluminum block may then be milled in a conventionalmanner to bring the aluminum to the predetermined height and flatness.While this approach has been used successfully to planarize fire decksof bimetallic engine blocks, the step of countersinking the cast ironsleeves disadvantageously adds an extra machining step, an extra toolchange and an extra tool set up which tends to increase the time andexpense of engine block fabrication. It is thus desirable to devise atool and/or process able to planarize the fire deck of a bimetallicengine block in a single pass or process step.

Another technique commonly utilized for metal removal involves use ofconventional grinding wheels, typically face grinding wheels or surfacegrinding wheels comprising alumina grain in resin bond. While thistechnique tends to be effective on cast iron workpieces, aluminum isrelatively soft, gummy and abrasive, and thus difficult to grind.Another drawback of this approach is that these bonded abrasive grindingwheels generally cannot be used in conventional milling machines, due todifferences in the parameters associated with milling versus grinding.One such difference is the need for guarding to protect machineoperators from debris expelled during use of the bonded abrasivegrinding wheel. This need for having discrete grinding machines inaddition to conventional milling machines, disadvantageously tends toincrease the cost of producing bimetallic engine blocks in this manner,due to increased overhead in terms of capital equipment costs andmanufacturing space requirements, etc.

Thus, a need exists for an improved tool and/or method for machiningfire decks of bimetallic engine blocks.

A significant reason for the difficulty associated with millingbimetallic workpieces is that during the milling operation, each bladeor insert of the milling head is maintained in relatively interruptedcontact with the bimetallic block, in which the blade repeatedly takesrelatively large cuts across the boundary between the soft aluminum andthe brittle cast iron as the milling head rotates. The relatively largenumber of cutting points provided by each abrasive grain of a grindingwheel provides a more continuous contact with the workpiece, in whicheach grain takes a relatively smaller cut or bite as it crosses theboundary between materials.

SUMMARY OF THE INVENTION

According to an embodiment of this invention, an abrasive insert isadapted for use on a milling head of a milling machine which includes amilling head adapted for rotation about a central axis of the millingmachine for machining operations. The abrasive insert includes asubstrate adapted for being mounted along a circumference of the millinghead. The substrate has a face of substantially planar configuration,adapted to extend orthogonally, radially outwardly from the centralaxis, and terminate at a radiused portion of substantially convex axialcross-section. An abrasive element disposed on the face is chosen fromthe group consisting of: metal brazed single layer abrasive elements;electroplated single layer abrasive elements; and abrasive elementscomprising grain bonded in a porous matrix having about 55 to 80 volumepercent interconnected porosity. During rotation of the milling head,the abrasive insert forms a notional annular grinding face for grindinga workpiece.

Another aspect of the present invention includes a grinding wheeladapted for machining a bimetallic workpiece. The grinding wheelincludes:

(a) a head adapted for rotatation about a central axis;

(b) a plurality of discrete abrasive inserts removably fastened inspaced relation along a circumference of the head;

(c) the plurality of discrete abrasive inserts each including:

(i) a metallic substrate having a face of substantially planarorientation, the face adapted to extend orthogonally, radially outwardlyfrom the central axis, and terminate at a radiused portion ofsubstantially convex axial cross-section; and

(ii) metal brazed single layer abrasive elements disposed on the face;

(d) wherein the plurality of abrasive inserts are individuallyreplaceable on the head.

A still further aspect of the invention includes a method for using amilling machine to grind a fire deck of a bimetallic engine block, themethod comprises the steps of:

(a) providing a milling head adapted for being rotated about a centralaxis by the milling machine, the milling head having a plurality ofinsert receiving receptacles disposed in spaced relation along acircumference thereof;

(b) providing a plurality of abrasive inserts adapted for releasablereceipt within the plurality of insert receiving receptacles, theplurality of abrasive inserts including an abrasive element disposedthereon, the abrasive element chosen from the group consisting of metalbrazed single layer abrasive elements, electroplated single layerabrasive elements, and abrasive elements comprising grain bonded in aporous matrix having about 55 to 80 volume percent interconnectedporosity;

(c) fastening said plurality of abrasive inserts in the plurality ofinsert receiving receptacles;

(d) orienting the axis of rotation at a predetermined angle α relativeto the fire deck;

(e) rotating the milling head about the axis of rotation, so that theplurality of abrasive inserts forms a notional annular grinding element;

(f) translating the milling head towards the engine block along a toolpath parallel to the fire deck, wherein the notional annular grindingelement engages and removes material from the block.

The present invention also may be adapted for use in finishing othersimilar components of vehicles, machines and the like.

The above and other features and advantages of this invention will bemore readily apparent from a reading of the following detaileddescription of various aspects of the invention taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational schematic view of a step in the process ofmachining a fire deck of an engine block according to the presentinvention;

FIG. 2 is a plan view of the process step of FIG. 1;

FIG. 3 is a plan view, on a reduced scale, of a milling head, includingan annular series of abrasive inserts, adapted for use during themachining process of FIG. 1;

FIG. 4 is a cross-sectional view taken along 4—4 of FIG. 3;

FIG. 5 is a plan view, on an enlarged scale, of an abrasive insert ofFIGS. 3-4;

FIG. 6 is an elevational view of the abrasive insert of FIG. 5;

FIG. 7 is a view similar to that of FIG. 3, including a pair ofconcentric annular series of abrasive inserts adapted for use on thegrinding wheel of FIG. 1;

FIG. 8 is a cross-sectional view taken along 8—8 of FIG. 7; and

FIG. 9 is an enlarged view of a portion of FIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Briefly described, the subject invention includes a method and apparatusfor machining flat or planarizing a fire deck 8 of a bimetallic engineblock 9 (FIG. 1). The invention includes providing a grinding wheel 10(FIG. 1) by disposing a series of abrasive inserts 11 (FIG. 3) at spacedlocations along a circumference of a circular head 16, so that duringrotation of the head, the inserts 11 form a notional annular grindingelement or ring 12 (FIG. 3). Head 16 may be a conventional milling headwith inserts 11 being sized and shaped to fit within receptacles adaptedto receive conventional milling inserts. In an alternate embodiment, asecond series of abrasive inserts 13 may be disposed radially inward ofring 12 to form an inner notional annular grinding ring 14 (FIG. 7)disposed concentrically with outer ring 12. In a preferred embodiment,inserts 11 and 13 each respectively comprise a single layer of diamondabrasive 18 (FIGS. 5 and 6) brazed onto a metallic substrate 20. Innerinserts 13 preferably extend further towards the workpiece in the axialdimension than outer inserts 11 (FIG. 7) so that inner ring 14 isprovided with greater height relative to the head 16 than the outer ring12.

Grinding wheel 10 is operated by orienting its axis of rotation 19(FIG. 1) at a predetermined oblique angle α relative to fire deck 8. Thewheel is then translated longitudinally towards and along engine block 9along a tool path 26 parallel to the fire deck wherein ring 12 willengage the block for bulk material removal, followed by inner ring 14(if utilized) which removes a smaller amount of material to apply therequisite surface finish to fire deck 8. Fabrication of inserts 11 and13 with a single layer of abrasive brazed onto a substantially planargrinding face may enable a conventional milling head and milling machineto be used to grind bimetallic workpieces without the need for theguarding typically required for bonded abrasive grinding wheels.

Throughout this disclosure, the term “axial” when used in connectionwith a portion of a grinding wheel, shall refer to a directionsubstantially parallel to axis of rotation 19 as shown in FIG. 1.Similarly, the term “radial” refers to a direction perpendicular ororthogonal to the axial direction.

Referring now to the Figures in detail, as shown in FIGS. 1 and 2, thesubject invention forms a grinding wheel 10 usable in combination with aconventional grinding or milling machine 11 to machine the fire deck 8of an engine block 9. Wheel 10 is configured to have an industrystandard (ANSI) Type 6, or flat cup shape, with notional outer annulargrinding element 12 disposed concentrically on head 16 to comprise thelip of the cup. As shown, grinding wheel 10 is utilized in aconventional face grinding manner, in which its axis of rotation 19 isoriented at a predetermined oblique angle α relative fire deck 8. Whilemaintaining angle α constant, the wheel is translated or moved alongtool path 26 to engage and machine block 9, including aluminum blockportion 28 and cast iron sleeves 30, to a predetermined height 32. Inone embodiment as shown, angle α is approximately 88 or 89 degrees.Alternatively, wheel 10 may be used in any number of operating modes,such as conventional multiple pass, orbital path, etc. Also, angle α maybe 90 degrees (not shown) to orient element 12 parallel to fire deck 8,in which diametrically opposed portions of element 12, may contact thefire deck simultaneously.

Turning now to FIGS. 3-4, abrasive inserts 11 are disposed at spacedlocations along a circumference of head 16, so that during rotation ofthe head, the inserts 11 form the notional annular grinding element orring 12. In a preferred embodiment, head 16 is a conventional millinghead and inserts 11 are sized and shaped to fit within receptaclesadapted to receive conventional milling inserts.

It was discovered that utilizing a plurality of discrete inserts 11spaced from one another to provide gaps therebetween as shownfacilitates delivery of coolant to the workpiece and removal of debrisor grinding swarf and helps avoid scratching the surface of theworkpiece. This aspect is particularly important when using single layerabrasive (as discussed hereinbelow) on a workpiece that is difficult tomachine or gummy, such as aluminum, and enables the present invention toovercome the aforementioned problems commonly associated with grindingaluminum. For this reason, grinding face 17 of each insert 11 isdisposed at a predetermined axial distance d from the head 16, so thatspacing between each segment serves to form a gap or slot 34 betweenadjacent inserts 11.

Preferably, as shown, each insert 11 includes a throughbore 21 tofacilitate releasable mechanical engagement with head 16, i.e. usingbolts or screws. Shim stock or the like may be conveniently utilized tofacilitate height adjustment and/or runout correction of each insert 11relative to the head. In this regard, such fastening and adjustment maybe advantageously simplified by fabricating inserts 11 in as fewdiscrete parts as possible. For example, two or more inserts 11 may befabricated as a single piece extending along a predetermined portion ofthe circumference of head 16. Moreover, it may be desirable to fabricateall of the inserts 11 as a one piece ring, sized and shaped to mate withthe insert receiving recesses of head 16, while providing substantiallythe same profile as shown in FIGS. 3 and 4, i.e. grinding faces 17separated by gaps of axial depth d to effectively form slots 34 disposedtherebetween. Similarly, ring 12 may be fabricated as a multi-partassembly, such as in two semicircular, 180 degree portions, four 90degree portions, or some other configuration in order to prevent orameliorate the accumulation of stresses and distortion due to highrotational speed testing. Moreover, inserts 11 may be fabricatedutilizing either a single layer of abrasive on a segmented metallicsubstrate, as discussed in greater detail hereinbelow, or by utilizing aporous bond matrix such as vitrified bonded abrasive segments. Althoughmechanical fastening as described above is preferred to facilitatereplacement of worn inserts in the manner common to milling inserts, theinserts of the present invention may be fastened to a head 16 in anysuitable manner, including brazing, welding and the like.

Turning now to FIGS. 5 and 6, inserts 11 preferably comprise a singlelayer 18 of diamond abrasive bonded in a bronze braze on the face 17 ofmetallic substrate 20. As shown in FIG. 6, face 17 is substantiallyplanar and adapted to extend orthogonally, radially outwardly from thecentral axis 19 (FIG. 1) of the head, and terminate at a radiusedportion or chamfer 36 of substantially convex axial cross-section.Chamfer 36 has a radius of curvature of at least 2-5 times the size ofthe abrasive grain to help prevent shearing of the grain from thesubstrate during use. In general, radii of curvature greater than thisminimum size is preferred. The insert is preferably sized and shaped forreceipt within insert receptacles of a conventional milling head. In oneembodiment, for example, an insert 11 is provided with dimensions ofapproximately 0.25 in (0.63 cm) in the axial direction (FIG. 6), and 0.5in (1.3 cm) in each of the tangential direction t and the radialdirection r as shown in FIG. 5. As also shown in FIG. 5, the terminalportion of face 17 is also preferably provided with a radius ofcurvature 23 in a radial plane, i.e. a plane orthogonal to axis 19 (FIG.1), which approximates the radius of curvature of the notional ring 12formed by the insert. Chamfer 36 serves to provide a smooth engagementof grinding wheel 10 with the workpiece and avoid scratching,particularly when wheel 10 is operated at an oblique angle α as shown.

Although a bronze bond and diamond abrasive are preferred, a wide rangeof acceptable bond materials and abrasive grains may be utilized. Inparticular, substantially any single layer of abrasive may be used.Moreover, any suitable bond may be used to secure the abrasive to thesubstrate. For example, electroplating may be used, however, theelectroplated bond tends to be weaker than the brazed bond, resulting inshorter tool life. In addition, abrasive grains may be lost from theelectroplated tool during grinding and the loose grains tend to score orscratch the workpiece. In such single layer abrasive wheels, the heightof the abrasive (in the direction orthogonal to face 17) should be keptnearly uniform to minimize wheel “runout.” The wheel can be finished tosubstantially reduce any runout by conventional grinding or machining toeliminate protruding grains and/or by using shim stock as will bediscussed hereinafter. Advantageously, wheels comprising a metallicsubstrate 20 with a single layer of abrasive 18 generally do not requireconventional truing or dressing and thus are preferred. This preferredembodiment is shown and described herein. In addition, however, openstructure face grinding inserts that utilize a highly porous bondmatrix, such as inserts having about 55 to 80 volume percentinterconnected porosity may be used. Inserts comprising conventionalvitrified bond are preferred for creating a porous matrix havingsufficient strength and tool life to grind bimetallic components.Interconnected porosity, and a permeability test useful for determiningthe porosity as a volume percent is disclosed in U.S. patent applicationSer. No. 08/687,816, which is fully incorporated by reference herein.

It was discovered that such open structure or porosity facilitatesdelivery of coolant to the workpiece and removal of debris or grindingswarf and helps avoid scratching the surface of the workpiece. Thisaspect is particularly important when the workpiece is difficult tomachine or gummy, such as aluminum, and enables the present invention toovercome the aforementioned problems commonly associated with grindingaluminum. Moreover, for this reason, in single layer abrasive wheels aplurality of radially extending slots 34, such as formed by spacing theinserts along the circumference of the head, are provided to furtherfacilitate swarf and coolant flow.

Wheels 10 fabricated according to the subject invention advantageouslyenable planarization of fire deck 8 in a single pass. Moreover, wheelperformance in a particular application may be further enhanced byadjusting certain wheel parameters. In this regard, wheel 10 shouldpreferably have an outer diameter dO (FIG. 1) diameter at least as largeas the width w (transverse to tool path 26), (FIG. 2) of the workpiece.For example, an outer wheel diameter dO of 28-30 cm is preferred for anengine block having a width w of 25 cm. In a particularly preferredembodiment, both outer diameter dO and inner diameter dI (FIG. 1) aregreater than width w to facilitate swarf and coolant flow, particularlywhen wheel 10 is operated with a 90 degree angle α. This sizing alsohelps prevent loading problems between the wheel and workpiece. Anotherconsideration with regard to wheel performance is abrasive grit size.Abrasive grit size utilized in layer 18 thus may be chosen by balancingsurface finish with wheel life. In this regard, smaller grit sizes tendto produce fewer burrs and surface defects, but tend to promote shorterwheel life. For a single ring wheel, diamond grit sizes of about 20 to50 are preferred. Conventional abrasive grit sizes of about 80 to 120are preferred.

As mentioned hereinabove, ring 12 should have a runout of less than 50microns over the abrasives. In a preferred embodiment utilizing a singlelayer 18 of abrasive, as long as substrate 20 is true, approximately 10%of the maximum abrasive diameter may be ground off using a resin bondeddiamond wheel to correct any runout in the layer 18. This translates togrinding as much as approximately 0.003″ from a layer of 20/25 meshabrasive and 0.0016″ from a 40/45 mesh abrasive.

Turning now to FIGS. 7-9, in an alternate embodiment, a second series ofabrasive inserts 13 may be disposed radially inward of ring 12 to forman inner notional annular grinding element or ring 14 (FIG. 7) disposedconcentrically with outer ring 12. Inserts 13 are preferably fabricatedin a manner similar to that of inserts 11, utilizing the same ordifferent abrasive grain size, as will be discussed hereinafter.Moreover, two or more inserts 13 may be fabricated integrally in themanner discussed above with respect to inserts 11. Inserts 11, includingtheir respective substrates 20 and 24, are fabricated to be discretefrom inserts 13. In this manner, they are individually fastened to head16 (FIG. 1) to facilitate independent height adjustment of elements 12and 14, such as with shim stock, to provide a predetermined axial heighth2 (FIG. 9) therebetween. Height h2 is determined based on the grit sizeof abrasive used on each series of inserts 11 and 13.

Thus, during grinding operation in the manner described hereinabove withrespect to FIGS. 1 and 2, outer ring 12 will engage the block for themajority of material removal, followed by inner ring 14 which serves toremove a smaller amount of material to eliminate any burrs or othersurface imperfections, etc. generated by the outer element and to applythe requisite surface finish to fire deck 8.

This double-ring embodiment enables the use of grit sizes more closelyoptimized for finishing bimetallic block 9. Thus, a relatively coursegrit may be utilized on outer ring 12 to efficiently remove therequisite amount of metal, and a finer grit used on inner ring 14 toprovide the fire deck with the desired surface finish. Thisconfiguration may advantageously improve wheel efficiency for improvedwheel life. For example, the diamond grit size used on outer ring 12 maybe 20-40 mesh, or larger, while the inner grit size may be 100-120 meshor smaller. The amount of material removed by inner wheel 14 is afunction of height h2, by which the inner wheel extends closer to theworkpiece than outer wheel 12 during its pass over block 9. This heightmay be approximately 20-40 microns.

The resulting surface finish utilizing a wheel of this embodiment is afunction of the radial distance between inner ring 12 and outer ring 14,the surface area of contact between each ring and the workpiece, thegrit sizes of the abrasive on each ring, and height h2 between each ofring 12 and 14.

In an additional aspect of this embodiment, inserts 11, including ansingle abrasive layer 18 on a metallic substrate 20 may be utilized asouter ring 12, in combination with a conventional matrix bonded abrasivegrinding wheel as inner ring 14. In a variation of this aspect, theinner wheel may be replaced with a cutting tool, by brazing ormechanically fastening one or more cutting tool inserts, i.e., CBN(Cubic Boron Nitride) or PCD (polycrystalline diamond) to the wheelradially inwards of outer ring 12. The tool inserts are preferablyprovided with a zero to negative rake, a chamfered cutting edge, and aslight, about 5° C., clearance angle at the rear of the cut. The purposeof the inserts is to remove as little material as possible but to leavea smooth surface finish.

The grinding wheels of the present invention thus have a relativelylarge number of cutting points provided by each abrasive grain of agrinding wheel. The wheels provide a relatively continuous contact withthe workpiece and take smaller cuts or bites from the workpiece. Thisserves to smooth the transitions between the hard phase of the cast ironcylinder liners 30 and the soft phase of the aluminum block 28 (FIG. 1).Better flatness or planarity and surface finish have thus been observedwith the grinding process of the present invention relative to the priorart milling processes.

The following illustrative examples are intended to demonstrate certainaspects of the present invention. All of the wheels in the Examples aretype 6, cup shaped wheels of the type shown in FIG. 1, with an 8 in (20cm) outer diameter. All the tests contemplate grinding a 7 inch (18 cm)aluminum/cast iron bimetallic engine block of the type describedhereinabove. These tests are summarized in Table I.

TABLE I Maximum Material Removal Rates Power (at Maximum Wheel Samplemaximum MRR Feed Rate Depth of Cut Examples 1-16 MRR) (inches³/min)(inches/min) per Pass 1 Control 9.28 0.25 20 0.005 2 Control 7.36 0.2520 0.005 3 Control 6.88 2.50 70 0.014 4 Exp. 6.56 3.16 90 0.014 5 Exp.5.92 3.86 110 0.014 6 Exp. 6.4 2.10 60 0.014 7 Exp. 6.8 1.76 50 0.014 8Control 1.6 1.00 20 0.02 9 Control 1.76 1.00 20 0.02 10 Control 5.281.00 20 0.02 11 Control 6.24 1.00 20 0.02 12 Control 11.04 0.75 15 0.0213 Control 7.2 0.63 50 0.005

Grinding Conditions

Okuma Machining Center (10 HP), with vertical spindle, CNC controlled

External coolant pump (20 psi)

Master Chemical E210 water soluble coolant at 10% in water, 30 gal./min

Wheel speed—3,000 rpm

Workpiece feed rate and depth of cut—See Table I

All conventional abrasive wheel rims were 1 inch wide

Superabrasive wheel 7 was 0.2 inch wide; all other superabrasive wheelswere 0.08 inch wide.

As shown, Examples 4-7 of the present invention are expected to providesubstantially improved material removal rates relative to control wheels1-3, 8-13. Wheels of the invention are expected to yield materialremoval rates at least comparable to the rates achieved by millingoperations used in the art. The flatness and surface finish achievedwith the wheels of the invention is expected to be superior to thatpossible in a milling operation or with electroplated wheels over toollife. Moreover, although surface flatness and finish are expected to beacceptable for all wheels tested, finish was better with wheels havingwider rims (e.g., for wheel 7, with a width of about 2 times the widthof wheels 4-6, there may be a 100 times decrease in surface roughnessunits (R_(a) μinch)). At material removal rates over about 3 in³/min,surface finish begins to degrade and power draw begins to decrease. Atrates below 3 in³/min, brazed single layer diamond tools (4-7) give thebest surface results (the diamond cuts freely, relative to conventionalabrasives, and there is no discernible grain loss to scratch thesurface). It is to be understood that these examples should not beconstrued as limiting.

EXAMPLES 1 AND 2

Control Wheels—Vitrified bonded diamond wheels with less than 55%porosity.

EXAMPLE 3

Control wheel—30/40 grit size diamond in electroplated metal bondedsingle layer diamond wheels with slots cut into the steel core of thewheel.

EXAMPLE 4

Invention wheel—20/25 grit size diamond bonded in 77/23 Cu/Sn bronzebraze. The wheel may be made by applying a paste containing the metalpowder of the braze in an organic binder to the inserts, applying thediamond to the paste, and then brazing the wheel at about 800-900° C.

EXAMPLE 5

Invention wheel—20/25 grit size diamond bonded as in Example 4 with asingle ring of inserts spaced to provide slots in about 20% of the areaof the rim.

EXAMPLE 6

Invention wheel—30/35 grit size diamond bonded and made as Example 4.

EXAMPLE 7

Invention Wheel—30 grit size diamond bonded as a single layer on steelinserts with a silver/copper braze at above 900° C. The abrasive may beapplied to the individual inserts and brazed, and finished insertsattached to the steel core backing in spaced relation to provide slotsbetween the inserts.

EXAMPLE 8

Control wheel—80 grit size sol gel microcrystalline alpha-aluminafilamentary grain, having a length:width aspect ratio of 4:1, madeaccording to U.S. Pat. No. 5,244,477 to Rue, et al and sold under theNorton Targa® trademark. The wheels have a vitrified bond and a totalporosity of about 57%, including 41% interconnected porosity and 16%closed cell (bubble alumina) porosity.

EXAMPLE 9

Control wheel—Same as Example 8 with 120 grit size filamentary abrasivegrain.

EXAMPLES 10 AND 11

Controls—Commercial products (phenolic resin bonded mix of fused aluminaand silicon carbide grains) conventionally used for face grinding ofmetals. The wheels have a porosity of about 20-40 volume %.

EXAMPLE 12

Control Wheel—37 grit size silicon carbide grain bonded in a vitrifiedmatrix with a porosity of less than 55% (about 30-35%).

EXAMPLE 13

Control Wheel—39 grit size silicon carbide grain bonded in a vitrifiedmatrix with a porosity of less than 55% (about 30-35%).

The foregoing description is intended primarily for purposes ofillustration. Although the invention has been shown and described withrespect to an exemplary embodiment thereof, it should be understood bythose skilled in the art that the foregoing and various other changes,omissions, and additions in the form and detail thereof may be madetherein without departing from the spirit and scope of the invention.

Having thus described the invention, what is claimed is:
 1. An abrasiveinsert adapted for use on a milling head of a milling machine, themilling head adapted for rotation about a central axis of the millingmachine for machining operations, said abrasive insert comprising: asubstrate adapted for disposition along a circumference of the millinghead; said substrate having a face of substantially planar orientation,said face adapted to extend orthogonally, radially outwardly from thecentral axis, and terminate at a radiused portion of substantiallyconvex axial cross-section; an abrasive element disposed on said face,said abrasive element chosen from the group consisting of metal brazedsingle layer abrasive elements, electroplated single layer abrasiveelements, and abrasive elements comprising grain bonded in a porousmatrix having about 55 to 80 volume percent interconnected porosity;wherein said abrasive insert forms a notional annular grinding faceduring rotation of said milling head, for grinding a workpiece.
 2. Theabrasive insert as set forth in claim 1, wherein said radiused portionis substantially convex in a radial cross-section thereof.
 3. The insertas set forth in claim 1, further comprising a metal brazed single layerabrasive element.
 4. The insert as set forth in claim 3, furthercomprising a metallic substrate and said abrasive element comprises asingle layer of diamond abrasive grains brazed onto said metallicsubstrate.
 5. The insert as set forth in claim 4, wherein said abrasivecomprises diamond grains have a grit size within a range ofapproximately 20-120.
 6. The insert as set forth in claim 4, whereinsaid single layer of abrasive is brazed onto said metallic substratewith a bronze braze.
 7. A grinding wheel adapted for machining abimetallic workpiece, said grinding wheel comprising: a head adapted forbeing rotated about a central axis for machining operations; and atleast one abrasive insert as set forth in claim 1, disposed on saidhead; wherein said grinding wheel is a cup wheel adapted for grinding abimetallic workpiece.
 8. The grinding wheel as set forth in claim 7,further comprising a plurality of abrasive inserts disposed at spacedlocations along the circumference of the milling head.
 9. The grindingwheel as set forth in claim 7, further comprising at least one otherabrasive insert disposed radially inward of said at least one abrasiveinsert to form a second notional annular grinding face during rotationof said milling head, said second annular grinding face being disposedat a predetermined height in the axial direction closer to thebimetallic workpiece than that of said first notional annular grindingface, wherein said second notional annular grinding face removesmaterial from the bimetallic workpiece after said first notional annulargrinding face, so that said second notional annular grinding face isadapted to apply a surface finish to the bimetallic workpiece.
 10. Thegrinding wheel as set forth in claim 9, further comprising a pluralityof said at least one abrasive inserts and a plurality of said at leastone other abrasive inserts.
 11. The grinding wheel as set forth in claim10, wherein said at least one abrasive inserts and said at least oneother abrasive inserts are individually fastenable to said head tofacilitate independent height adjustment of the elements in said axialdirection relative one another.
 12. The grinding wheel as set forth inclaim 9, wherein said at least one other abrasive insert comprises anabrasive of a type distinct from that of said at least one abrasiveinsert.
 13. A grinding wheel adapted for machining a bimetallicworkpiece, said grinding wheel comprising: (a) a head adapted forrotatation about a central axis; (b) a plurality of discrete abrasiveinserts removably fastened in spaced relation along a circumference ofsaid head; (c) said plurality of discrete abrasive inserts eachincluding: (i) a metallic substrate having a face of substantiallyplanar orientation, said face extending orthogonally, radially outwardlyfrom the central axis, and terminating at a radiused portion ofsubstantially convex axial cross-section; and (ii) metal brazed singlelayer abrasive elements disposed on said face; (d) wherein saidplurality of abrasive inserts are individually replaceable on said head.